This invention relates generally to safety and safety relief valves of the spring loaded self-actuated type, that is, relief valves for pressurized systems in which the valve member is biased toward its seat by a spring force oppposed to the system pressure acting on the valve member. More particularly, the invention relates to apparatus and systems adapted for assisting the functions of the relief valve and for measuring its set pressure.
In my U.S. Pat. No. 4,349,885 dated Sept. 14, 1982, I have described a set pressure measuring system for safety relief valves wherein pneumatic pressure is applied to a bellows adapted for the application of a force to the spindle of the relief valve. A controller is adapted for pressurizing the bellows with the relief valve installed and operating in the pressurized system at a pressure below the set pressure. By "set pressure," which is also referred to as the self-actuation pressure, is meant the system pressure at which the pressure force on the relief valve tending to open it just equals the bias spring force holding the valve against the seat. According to said patent, the pressure applied to the bellows is elevated to a value sufficient to produce a force which, when added to the force of the then-existing system pressure, will equal the bias spring force on the valve, allowing the valve to lift open. The latter event, called the "test time," is detected by a linear variable differential transformer or LVDT, which is also connected to the controller. The controller is further provided with means to measure the system pressure and the force applied by the bellows to the valve spindle at the test time. Computer functions associated with the controller then derive the actual set pressure of the relief valve as the sum of the system pressure at the test time and a computed increment of the system pressure that would provide a force equal to the force applied by the bellows at the test time. In this manner the apparatus of said patent has provided many advantages in its primary function of measuring the relief valve set pressure and detecting any drift therein while in service. Such advantages include the ability to verify or measure the set pressure from a remote location with the valve in place and the pressurized system in operation. This reduces the risk of operator exposure when the relief valve is installed in a hazardous environment, as in a boiling water nuclear power plant, for example. Typically, it takes only one operator a matter of minutes to verify the set pressure on a large number of relief valves without interfering with the over-pressure protection function of the valves. In practice, the system has produced great economies and reduced the need for system shut-down, without sacrifice of the safety precautions required by regulatory agencies.
In addition to measuring the set pressure of a safety relief valve and detecting any drift therein, it has been found desirable to ensure that in operation the relief valve remains fully seated until the system pressure reaches the value at which the valve is to open, that the valve does open at the desired system pressure, and that the valve closes at the desired blowdown pressure valve. A common problem particularly with steam safety valves is that they leak or "simmer" at pressures somewhat below the set pressure. Also, it is extremely important to ensure a precise blowdown especially in relief valves for nuclear systems to limit the release of system fluid.
Accordingly, one of the objects of the present invention is to provide a function assist control for a pressure relief valve that will ensure its opening at a predetermined pressure notwithstanding any drift in its set pressure.
A second object is to provide a function assist control, also for opening the pressure relief valve, which may be used to demonstrate its full relieving capability at a time when the system pressure is actually well below the valve set pressure, for example as low as 75 percent of the set pressure.
A third object of the invention is to provide a function assist control for urging the valve toward its seated position upon reaching the blowdown pressure.
A fourth object of the invention is to provide an anti-simmer control that is selectively operative to hold the relief valve against the seat during normal system operation.
A fifth object is to accomplish the foregoing function assist controls by means that will also permit the use of the set pressure measuring system of said patent from a remote location, so that an operator is not required to enter a hazardous containment where the relief valve may be located.
With the above and other objects hereinafter appearing in view, the features of this invention include a concentric dual-bellows, externally mounted function assist control for a spring-loaded, self-actuated safety relief valve. An outer bellows functions in a manner similar to the bellows described in said patent for applying a force to the valve spindle tending to open the valve. A second bellows is provided for applying, under predetermined operating conditions, a force to the spindle tending to close the relief valve.
This bellows arrangement, when associated with a suitable controller adapted to respond to the system pressure and to control the pressurization and depressurization of the bellows selectively, provides great flexibility in the application of the function assist controls to the valve.
Further features of the invention comprise other elements of the apparatus adapted to perform the functions described in said patent including the measurement or verification of the set pressure at which the relief valve will be self-actuated.